Watermarks have long been used in the printing industry to identify the source or origin of a document. Generally, a watermark appears as a faint pattern in an image, which is visible only when the original document is viewed in a particular manner. Unless a counterfeiter had access to the watermarked paper, it would be difficult for him to reproduce the document without showing its inauthenticity. That is to say, without the paper on which the original image was originally printed, the copy should be readily detectable. However, as people move away from the use of watermarked papers for cost and other practical reasons, it is still necessary to identify the source or origin of a document image.
The introduction of the plain paper copier has resulted in a proliferation of paper copies of paper originals. A similar result is happening to electronic images, given the easy availability of digital scanners and quick and widespread access to images throughout the Internet. It is now very difficult for the creator of an image to generate an electronic original, for which he can be assured that illegal copies will not be spread to third parties. The use of a digital watermark is a technology that aims to prevent that spread, by incorporating an identifying mark within an image that allows one to identify the source of the image in an electronic copy. It is important that the identifying mark not be disturbing or distracting to the original content of the image, while at the same time, allowing easy identification of the source. The watermarks could be added either by the scanner, as or after the image is acquired, or by the halftoning software.
Watermark identification may be accomplished by embedding a digital watermark in a digital or printed page that will identify the owner of rights to the image. Watermarking can take two basic forms, visible or perceptible, and invisible or imperceptible. Visible watermarks are marks such as copyright logos or symbols or logos that are imprinted into the digital or printed image to be distributed. The presence of the watermark is made clearly visible in the image or rendered document in a way that makes it difficult to remove without damaging the image or document. The presence of the visible watermark does not harm the usefulness of the image, but it deters the use of the image without permission. However, visible watermarks may interfere with the use of the image or with the image aesthetics. The visible watermark is also a potential target for fraud, in that it is possible for a fraudulent copier of the image to identify the location of the watermark and attempt to reproduce the image without the watermark or try to transfer the watermark to another image.
Invisible watermarks are marks such as copyright symbols, logos, serial numbers, other identifiers, etc. that are embedded into digital or printed images in a way which is not easily discernible or perceptible by the unaided eye. At a later time, the information embedded in these watermarks can be derived or “retrieved” from the images to aid identification of the source of the image, including the owner and the individual to whom the image is sold. Such watermarks are useful for establishing ownership when ownership of an image is in dispute. They would be less likely to be useful as a deterrent to the theft of the image. While either or both visible or invisible watermarks are desirable in an image, they represent different techniques for either preventing copying or detecting copying. It is anticipated that document producers may wish to use both kinds of protection.
Embedded watermarks in printed halftone images, which can subsequently be detected using a visual aid or using a watermark detection algorithm on a scan of the image are of interest in wide range of applications. U.S. Pat. No. 5,790,703 to Shen-ge Wang for “Digital Watermarking Using Conjugate Halftone Screens,” describes a method for generating watermarks in black and white halftone printing using stochastic screens. In U.S. Pat. No. 5,790,703, monochrome digital watermarks are embedded as correlations in the halftone screen. U.S. Pat. No. 6,731,409 to Shen-ge Wang for “System and Method for Generating Color Digital Watermarks Using Conjugate Halftone Screens” extends the method of U.S. Pat. No. 5,790,703 to color printing. The method disclosed in U.S. Pat. No. 6,731,409 generates color watermarks by producing a halftone pattern in one or more color separations of the color document using a separate halftone screen for each separation. While the color contrast watermarks produced by U.S. Pat. No. 6,731,409 work well on digital bit-maps, the color watermarks are harder to detect in printed hardcopy. The contrast and signal-to-noise ratio of the color watermark produced can be quite weak because of screen interactions making it impractical for high resolution color printing applications. It is also harder to determine the shift required for performing a watermark estimation when detecting the watermark pattern in scanned images due to the use of different screens for the different color separations. What is needed is a method of generating color watermarks having good contrast and good signal-to-noise ratios for detection on scans of printed images.
Disclosed in embodiments herein is a method for generating color digital watermarks, which provides good contrast while maintaining good signal-to-noise ratios. Instead of using a separate screen for each separation, the method uses a single halftone screen for all color separations. In one embodiment, the method uses a stochastic halftone screen with an embedded watermark, i.e., a stochastic screen with a conjugate relationship to describe the location of the watermark. The conjugate relationship determines the placement or location of the dots corresponding to the digital watermark. A successive fill technique, such as the one described in U.S. Pat. No. 6,844,941 to G. Sharma et al. for Color Halftoning Using a Single Successive-Filling Halftone Screen, the contents of which are incorporated herein by reference in their entirety, may be used to color the dots forming the digital watermark within the halftone screen. For example, if an image has four separations, the successive fill technique will determine whether to color a particular dot black, cyan, magenta, yellow or not at all. By using a single halftone screen and a successive fill technique, the different separations work together to produce the watermark with a significantly higher signal to noise ratio. In addition, the use of a single screen significantly improves the synchronization for the watermark detection process.
For purposes of this disclosure, the term “screening” or “halftoning” refers to the process in which each pixel value of a 2D array of contone pixels is compared to one of a set of preselected thresholds (the thresholds may be stored as a 2D matrix and the repetitive pattern generated by this matrix is considered a halftone cell), which produces a binary output at each pixel according to the result of the comparison. The matrix of threshold values is often referred to as a “screen”, and the process of generating the binary image from the contone image using the screen is called “screening” or halftoning.
Also disclosed herein is a method for generating an authenticable color image, the color image including a plurality of color separations, wherein an authenticable image inserted in the color image is not readily visually perceptible. According to one embodiment, the method includes providing a single halftone screen, wherein the single halftone screen comprises a plurality of pixel locations with associated threshold values; wherein the halftone screen has a plurality of cells, each cell having a first region and a second region, each cell being spatially offset from a neighboring cell by a first distance; wherein a first region of a first cell is substantially identical to a first region of a second cell, and a second region of the first cell is substantially conjugate to a second region of the second cell; halftoning image data corresponding to a first color separation using the single halftone screen, wherein a corresponding first set of screen pixel locations associated with a first set of threshold values are filled by the first color separation; halftoning image data corresponding to a second color separation using the single halftone screen, wherein a corresponding second set of screen pixel locations are filled by the second separation, the second set having threshold values successive to the first set of threshold values. When a first copy of the color image is spatially offset from a second copy of the color image by at least the first distance, at least a first cell of each of the first and second copy of the color image align with at least a second cell of the first and second copy of the color image, and contrast of the identical and conjugate regions become visible to form the authentication image. The single halftone screen may be a stochastic screen.
The method can be expanded to cover placement and coloring of the third and fourth color separations. The method may further include halftoning image data corresponding to a third color separation using the single halftone screen, wherein a corresponding third set of screen pixel locations are filled by the third separation, the third set having threshold values successive to the second set of threshold values. For a fourth color separation, the method may further include halftoning image data corresponding to a fourth color separation using the single halftone screen, wherein a corresponding fourth set of screen pixel locations are filled by the fourth separation, the fourth set having threshold values successive to the third set of threshold values. The color separations may be black, magenta, cyan and yellow.
Further disclosed is a method for generating an authenticable color image, the color image including a plurality of color separations, wherein an authenticable image inserted in the color image is not readily visually perceptible. The embodiment includes providing a single stochastic halftone screen, the single stochastic halftone screen comprises a plurality of pixel locations with associated threshold values; wherein the stochastic halftone screen has a plurality of cells, each cell having a first region and a second region, each cell being spatially offset from a neighboring cell by a first distance; wherein a first region of a first cell is substantially identical to a first region of a second cell, and a second region of the first cell is substantially conjugate to a second region of the second cell; halftoning image data corresponding to the plurality of color separations using the stochastic halftone screen; further comprising, for each pixel, summing image values corresponding to the plurality of color separations in a predetermined order; comparing sums image values of at least the first and second color separations to the threshold values in the stochastic halftone screen; determining placement and color of the pixel in accordance with a predetermined relationship based on the comparison; wherein, when a first copy of the color image is spatially offset from a second copy of the color image by at least the first distance, at least a first cell of each of the first and second copy of the color image align with at least a second cell of the first and second copy of the color image, and contrast of the identical and conjugate regions become visible to form the authentication image.
For two separations, the predetermined relationship may be: if (i1>screen_threshold), printing a pixel with the colorant of the first separation; if ((i1+i2)>screen_threshold) and (i1<screen_threshold)), printing a pixel with the colorant of the second separation; and if ((i1+i2−M)>screen_threshold), printing a pixel with the colorant of the second separation; where i1, i2 are the image values of the image data for the first color separation and the second color separation, respectively, screen_threshold is the value of a threshold in the stochastic halftone screen, and M is the maximum threshold value. Note that with the use of this method a given pixel may be printed with none, one, or both of the colorants. The predetermined relationship may be extended to a third separation and include: if ((i1+i2+i3)>screen_threshold) and ((i1+i2)<screen_threshold)), printing a pixel with the colorant of the third separation; if ((i1+i2+i3−M)>screen_threshold) and ((i1+i2−M)<screen_threshold), printing a pixel with the colorant of the third separation; and if ((i1+i2+i3−M)>screen_threshold), printing a pixel with the colorant of the third separation.
A method for generating an authenticable color image, the color image including a plurality of color separations, wherein an authentication image inserted in the color image is not readily visually perceptible, according to another embodiment of the method for generating an authenticable color image, includes halftoning image data corresponding to a first color separation using a single halftone screen, wherein the single halftone screen includes means for generating an authentication image in a color image; and halftoning image data corresponding to a second color separation using the single halftone screen and dot placement information for the image data corresponding to the first color separation to form a multicolor image; wherein halftoning of image data corresponding to the second color separation includes placing dots for the second color separation in thresholds of the halftone screen relative to those thresholds occupied by the first color separation in the halftone screen in accordance with a predetermined relationship.
Examples of predetermined relationships include: placing thresholds for the second color separation adjacent to the thresholds of the first color separation, placing thresholds for the second color separation at a predetermined distance from the thresholds of the first color separation, and generating a modified value for the second separation by adding the “halftone error” from the first separation and obtaining the second separation by screening the modified value for this separation. For example, halftoning of image data corresponding to the second color separation may include placing halftone dots for the second color separation in thresholds of the stochastic halftone screen determined by: determining a halftone error between the first color separation image data and the second color separation image data; and adding the halftone error to the second color separation image data.